In a conventional method of sorting mail items in a plurality of passes by using a mail item identity code, also known as an “ID tag”, while the mail item is passing through the machine for the first time, a digital image is formed of the surface of the mail item that bears address information for performing an automatic address recognition operation by Optical Character Recognition (OCR), a mail item identity code is generated and that code is printed in the form of a bar code on the surface of the mail item. In a manner known per se, that code serves to retrieve sorting information from the database of the sorting machine in order to machine-sort each mail item during successive passes through the machine.
Such a bar code might not be read completely during the second pass of the mail item due to the bar code being printed with insufficient printing quality, or due to the particular type of the medium on which the bar code is printed. In particular, if the envelopes of the mail items are made of recycled paper (with a noisy or colored background), the symbols of the bar codes tend not to contrast very strongly with the background, and it is therefore possible that the bar code might be read incompletely. If the bar code cannot be read completely during a second sorting pass, the corresponding mail item must be removed (rejected) from the automatic sorting process because it is no longer possible to access the sorting information. As a result, the mail item must be sorted manually, which constitutes a time-consuming and costly process.
An automatic postal-sorting process is characterized by its error rate, which represents the quantity of mail items incorrectly handled by the automatic sorting process.